This invention relates generally to high power laser sources and, more particularly, to midwave infrared (MWIR) high power laser sources. These laser sources are needed for a variety of applications, both military and commercial, but output power is significantly limited with current MWIR technology. Midwave infrared radiation is typically defined as having a wavelength between 2.5 to 6 μm. Near infrared (NIR) radiation has a wavelength between visible red and the midwave IR range, i.e., about 0.7 to 2.5 μm.
Phased arrays of high power fiber amplifiers have been demonstrated or proposed but have significant drawbacks. In a typical prior art approach, output from a master oscillator (MO) is distributed to an array of high power fiber amplifiers pumped by laser diode arrays. The output beams from the fiber amplifiers are combined in a closely packed lens array to form the output beam. A sample of each output beam is compared on a detector array, to a frequency shifted reference wavefront derived from the MO, to provide a measurement of the instantaneous phase of each fiber amplifier in the array, and the phases are then corrected in real time to form the output beam. The output from the MO defines the spectrum and modulation waveform input to the amplifiers. A critical limitation is that the wavelength of operation is restricted by the gain bandwidth of the rare earth dopant used in the core of the fiber amplifiers. For the most efficient designs this wavelength happens to fall in the region of 1000 nm to 1100 nm using ytterbium (Yb) as the dopant. Unfortunately, this is a spectral region in which the eye is quite vulnerable to permanent damage. While fiber laser arrays can be constructed at longer eyesafe wavelengths (e.g., beyond 1500 nm) using erbium-ytterbium (ErYb), Thulium (Tm), holmium (Ho), and other materials, the efficiency and wavelength coverage are not optimum. If factors relating to eye safety force the selection of a wavelength longer than 1500 nm for a laser weapon system or high power illuminator, for example, performance can be significantly compromised unless potentially efficient and scalable architectures can be developed.
The basic architecture of which the present invention is an improvement, is described in various prior patents, notably U.S. Pat. No. 5,694,408 to Bott et al., “Fiber Optic Laser System and Associated Lasing Method.” The present invention also utilizes a prior art technique for beam formation and phase control, as described in four other patents: U.S. Pat. No. 6,147,755 to Heflinger et al., “Dynamic Optical Phase State Detector,” U.S. Pat. No. 6,229,616 to Brosnan et al., “Heterodyne Wavefront Sensor,” U.S. Pat. No. 6,243,168 to Heflinger et al., “Dynamic Optical Micrometer,” and U.S. Pat. No. 6,366,356 to Brosnan et al., “High Average Power Fiber Laser System with High-Speed, Parallel Wavefront Sensor.” To the extent needed to provide a complete disclosure, these patents are incorporated by reference into this document.
It will be appreciated from the foregoing, that there is a need for a laser source that is both scalable to high powers and operable at a selected wavelength that is not restricted by the properties of dopants used in fiber amplifiers. The present invention satisfies this need and provides other advantages over the prior art.